Fix StereoTrigger non-deterministically discarding LST-1 in prod6 files

This was one of the weirder bugs I have ever tracked down.
The symptom was that in consecutive runs of `ctapipe-process` on the
same input file, sometimes LST-1 was missing from the output completely.

Tracking it down to the SoftwareTrigger class, which used a mixture of
using ``TelescopeDescription`` and its string representation for storing
and looking up things.

The dict `_ids_by_type` was the culprit, its content depended on the
hash of str(TelescopeDescription), which was random, since hash of
strings are random for security reasons in python.

docs/changes/2552.bugfix.rst

@@ -0,0 +1,6 @@
+Fix ``SoftwareTrigger`` not correctly handling different telescope
+types that have the same string representation, e.g. the four LSTs
+in prod6 files.
+
+Telescopes that have the same string representation now always are treated
+as one group in ``SoftwareTrigger``.

src/ctapipe/instrument/trigger.py

@@ -59,10 +59,16 @@ class SoftwareTrigger(TelescopeComponent):
     def __init__(self, subarray, *args, **kwargs):
         super().__init__(subarray, *args, **kwargs)
 
-        self._ids_by_type = {
-            str(type): set(self.subarray.get_tel_ids_for_type(type))
-            for type in self.subarray.telescope_types
-        }
+        # we are grouping telescopes by the str repr of the type
+        # this is needed since e.g. in prod6, LST-1 is slightly different
+        # from LST-2 to LST-4, but we still want the trigger to work with all
+        # LSTs
+        self._ids_by_type = {}
+        for tel in self.subarray.telescope_types:
+            tel_str = str(tel)
+            if tel_str not in self._ids_by_type:
+                self._ids_by_type[tel_str] = set()
+            self._ids_by_type[tel_str].update(self.subarray.get_tel_ids_for_type(tel))
 
     def __call__(self, event: ArrayEventContainer) -> bool:
         """
@@ -79,24 +85,22 @@ def __call__(self, event: ArrayEventContainer) -> bool:
         """
 
         tels_removed = set()
-        for tel_type in self.subarray.telescope_types:
-            tel_type_str = str(tel_type)
-            min_tels = self.min_telescopes_of_type.tel[tel_type_str]
+        for tel_type, tel_ids in self._ids_by_type.items():
+            min_tels = self.min_telescopes_of_type.tel[tel_type]
 
             # no need to check telescopes for which we have no min requirement
             if min_tels == 0:
                 continue
 
             tels_with_trigger = set(event.trigger.tels_with_trigger)
-            tel_ids = self._ids_by_type[tel_type_str]
             tels_in_event = tels_with_trigger.intersection(tel_ids)
 
             if len(tels_in_event) < min_tels:
                 for tel_id in tels_in_event:
                     self.log.debug(
                         "Removing tel_id %d of type %s from event due to type requirement",
                         tel_id,
-                        tel_type_str,
+                        tel_type,
                     )
 
                     # remove from tels_with_trigger

src/ctapipe/tools/process.py

@@ -301,7 +301,6 @@ def start(self):
             disable=not self.progress_bar,
         ):
             self.log.debug("Processing event_id=%s", event.index.event_id)
-
             if not self.event_type_filter(event):
                 continue